Color-blind people are bots.

[s0nido]

They use barely legible Captcha images because bots can recognise text in images if the text is legible enough. There is a voice option on most ofthese captcha images, though.

 

[waterpear]

The best captcha systems are the ones that use actual words (second one reads "dry") instead of nonsense (first one reads God knows what, probably "F5yyFdz". Those are easier for humans to read if there's noise, but they're still fairly bot-resistant.

They still exclude blind users, however. Adding sound would help...if sound clips were supported on all browser configurations. Making the sound clip run on command isn't guaranteed to work, because that would require scripting.